The present invention is directed to a sensing device or apparatus of a printing apparatus or device. The sensing device is mounted or coupled with a printing head of the printing device which printing head moves along the recording medium which is in turn conducted over a reflective surface. The sensing device includes a radiation source for projecting radiation at the recording medium and a photoelectric receiving means for sensing radiation which has been reflected by the reflective surface to determine whether or not the recording medium is present.
Sensing devices of the type in which light is reflected from a reflective surface and detected to determine the presence of a recording medium are generally known and have been successfully employed. An example of such a device is disclosed in German OS No. 29 51 934 and this device has a writing head which is moved line-wise along the recording medium and a sensing means, which includes a radiation source and a photoelectric receiving means, is provided in the writing head for sensing the edges of the recording medium. To eliminate problems from outside light, the sensing is done pulse-wise. The recording medium is conducted over a reflective surface which will reflect the light beam proceeding from the radiation source to the receiving means in those regions of the reflective surface that is not covered by the paper of the recording medium.
A light-emitting diode is employed as a radiation source. This light-emitting diode will emit a relatively highly focused light beam which is received by a photo-transistor after reflection by the reflective surface.
A significant problem with photosensors, however, consists therein that they are relatively sensitive to changes in the direction of the reflected light beam so that a slight departure of the reflective surface from its rated position leads to the fact that the sensing beam no longer reaches the receiver with full intensity. This will lead to a misinterpretation and misconnection in the region of the post-processing electronics under certain conditions. Since the reflective surface given such a device must be disposed in the region of the platen and the recording medium is conducted over the reflective surface, pivoting or, respectively, local distortions of the reflective surface, which will deflect or scatter light beams away from the photosensor, can easily occur.
Given the employment of a light-emitting diode and phototransistors in the sensing device, it is usual to proceed or follow the sensor surface or, respectively, the non-emitting surface with an optical system. For example, lenses are utilized in order to either focus the light to be emitted or on the other hand to concentrate the received light on the actual receptive surface of the receiver.
In the U.S. Pat. No. 3,739,177, for example, a sensing means for recognizing the edge of a paper web is disclosed. This sensing means has a lamp, which is provided as a radiation source. A convergent lens is allocated to the lamp and focuses the light proceeding from the lamp as it is supplied to the sensing region. At the receiver side, a convergent lens is in turn provided in front of the photosensitive element and collects the light reflected from the sensing surface and concentrates it on the receptor surface of the photosensitive element.